The present invention generally relates to a high matching precision organic light emitting diode (OLED) driver by using a current-cascaded method, and more particularly, to a method in which the current-cascaded method is used so as to reduce the driving current mismatching brought about by the drifting in the parameters during different fabrication processes, and thus improve the display quality.
Among the computer peripherals, displays serve as important output devices. Recently, due to the increasing demand of displays that are thin and light, thin film transistor-liquid crystal displays (TFT-LCD""s) have consequently been widely used. In addition, other thin and light displays and related techniques have been vastly investigated. In particular, the display drivers strongly affect the quality of display, and thus are very important.
To date, the most widely used dot-matrix displays are thin film transistor-liquid crystal displays (TFT-LCD""s), which utilize the voltage signals to control the ON/OFF state of the thin film transistor (TFT) and control the display color and brightness. During the past two years, organic light emitting diodes (OLED""s) represent a new display technique. In an organic light emitting diode (OLED), different organic molecules have different energy bandgaps, and accordingly lights of different energies, and consequently, colors are emitted as electrons from different conduction bands and holes from different valence bands recombine. In such a manner, organic light emitting diodes (OLED""s) can serve as light sources providing light of different colors and do not need a back light plate as thin film transistor-liquid crystal displays (TFT-LCD""s) do. Therefore, the aspect thickness and the fabrication cost of a display can be reduced.
Please refer to FIG. 1, which is a somewhat schematic cross sectional view illustrating the basic compositional structure of a conventional organic light emitting diode in accordance with the embodiment of the prior art. In the drawing, the structure comprises, from the top, a cathode 2, which is connected to the negative end of the electric source; an electron transport layer (ETL) 4; an emitter layer (EML) 6; a hole transport layer (HTL) 8; a hole injection layer (HIL) 10; an anode 12, which is connected to the positive end of the electric source; and finally a glass substrate 14 to complete the basic formation of an organic light emitting diodes (OLED). As for the flow directions of the electrons and the holes, please refer to FIG. 2, which is a somewhat schematic diagram illustrating the flow directions of the electrons and the holes across the electron transport layer (ETL) 4, the emitter layer (EML) 6, the hole transport layer (HTL) 8 and the hole injection layer (HIL) 10 in accordance with the embodiment of the prior art. To be more specific, the holes flow from the anode 12 through the hole injection layer (HIL) 10 and the hole transport layer (HTL) 8 to the emitter layer (EML) 6, as the electrons flow from the cathode 2 through the electron transport layer (ETL) 4 to the emitter layer (EML) 6, where the electrons and holes recombine and photons with energy equal to the energy difference between the conduction band and the valence band are emitted.
FIG. 3 is a somewhat schematic circuit diagram illustrating the driving system of dot-matrix display in accordance with the embodiment of the prior art. In the drawing, the diode symbols are implemented by using organic light emitting diode pixels 18, each of which is driven under the control of a current switch 17 and a state switch 19. The current switch 17 controls the input of the driving current 16, and the state switch 19 determines the pixel 18 to be connected either to the ground or to high level. In such a manner, a basic circuit structure of the driving system of a dot-matrix organic light emitting diode display is thus completed.
On the other hand, the brightness of the organic light emitting diode is controlled by the input current. Therefore, in order to achieve high brightness uniformity and high display quality, all the IC""s that drive the display are required to provide identical output currents. In other words, all the output currents are determined to match. For a high resolution organic light emitting diode display panel, a set of driving IC""s are connected in parallel to simultaneously provide the driving current. If the set of IC""s connected in parallel are driven under the control of voltage signals according to the conventional method, there occurs the output current mismatching of each driving IC brought about by the drifting in threshold voltage VT or offset voltage VOS of the operation amplifiers in each IC due to different fabrication processes. Accordingly, the display quality is affected.
For a detailed description of this problem, please refer to FIG. 4, which is a somewhat schematic layout diagram illustrating the internal circuit of a set of IC""s connected in parallel to drive the organic light emitting diodes under the control of voltage signals according to the conventional method of the prior art. As the external circuit delivers the voltage signals determined by the brightness of the same level to each driving IC, there occurs the difference between the output current Iout1 of the first driving integrated circuit IC1 and the output current Iout2 of the second driving integrated circuit IC2 brought about by the drifting in threshold voltage VT or offset voltage VOS of the operational amplifiers in each IC due to different fabrication procedures. The relation between the output current difference and the offset voltages of the operational amplifiers can be described as below:
Iout=(VBTxe2x88x92VOS)/Rxe2x80x83xe2x80x83(1)
xcex94Iout=Iout1xe2x88x92Iout2=(VOS2-VOS1)/Rxe2x80x83xe2x80x83(2)
From equations (1) and (2), we know the reason the driving currents according to conventional technique as shown in FIG. 4 mismatch is that the offset voltages of the operational amplifiers are difficult to be implemented to achieve complete matching. Moreover, the large difference between different IC""s can hardly be overcome due to the complicated internal circuit design of the operational amplifiers.
Furthermore, as shown in FIG. 4, the two resistors R1 and R2, are used in the external circuit; in practical applications, however, the two external resistors can hardly be implemented to be identical either. As a result, the mismatching problem occurs and needs to be solved. As for the mismatching problem of the current mirrors, the error is endurable and is not taken into consideration since it is a problem due to the in-chip IC layout and much less significant than the former two.
In order to overcome the problems that have been previously discussed above, the present invention has been proposed and relates to a method in which the current-cascaded method is used so as to reduce the driving current mismatching brought about by the drifting in the parameters during different fabrication procedures, and thus improve the display quality.
Accordingly, it is the main object of the present invention to provide a high matching precision organic light emitting diode (OLED) driver by using a current-cascaded method, in which the error resulting from the output current mirror mismatching can be determined by the error resulting from the in-chip IC process, instead of the errors from the external resistor mismatching and from the offset voltage difference between the operational amplifiers. Therefore, the image quality of the display can be improved by controlling the pixel driving currents to be stable and identical.
In order to accomplish the foregoing objects, the present invention provides a current-cascaded method for a plurality of driving IC""s of organic light emitting diodes that is different from the conventional method for driving IC""s in the design that the internal circuit of a first driving integrated circuit (IC) in accordance with the present invention comprises a first operational amplifier, which is used to receive an input voltage signal and then execute the signal amplification; a plurality of output transistors used as the output buffer transistors of said first operational amplifier are connected to one of the inputs of said first operational amplifier at the other end, wherein said first operational amplifier is enclosed in a closed loop and serves as an unity-gain buffer, wherein the outputs of said plurality of output transistors are further connected to said other plurality of driving IC""s so as to make the output currents of said other plurality of driving IC""s to be identical and match; a first current mirror, which is connected to said first output transistor so as to provide said other driving IC""s that are connected to said other output transistors with the current source; a resistor, which is externally connected to said first operational amplifier and said output transistors so as to further modulate the output current of said driving IC.